According to the dx517 specs on the Synology site:
“To have better performance, we recommend creating storage volumes physically located within the same NAS or Expansion Unit, rather than creating volumes across your NAS and Expansion Unit.”
I’m considering getting a dx517 as well so this was a nice rabbit hole to go down. I think your gut feel about the single esata cable is accurate.
Edit: And drive order shouldn’t matter. Obviously take a backup if you can but you should be able to power down, move the drives, and power back up and it continue to work without skipping a beat. At least using the default SHA RAID or w/e.